Car Battery failures

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ChrisJC
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Car Battery failures

Post by ChrisJC »

Hi All,
This lockdown has seen the batteries on both of my V8's (P38 Rangie and Landie) become fully discharged multiple times through lack of use.
Each time I have jump started the vehicle, and away we go.

However, both vehicles have managed to knacker their batteries.

I have been wondering why this is, and realise that jump starting a car with a flat battery is a really bad idea!! This is why:

The Landie battery is 60Ah, and this means the maximum charge current is 25% of that, so 15 Amps.

The alternator is a 150 Amp alternator, so it will stuff 150 Amps into a fully flat battery.

I think that doing this just a handful of times will completely knacker the battery.

I therefore think that if I get a flat battery, I must not use the vehicle, but recharge the battery with a normal charger first.

Any thoughts?

Chris.


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richardpope50
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Re: Car Battery failures

Post by richardpope50 »

Why don't you use a conditoning charger? Years ago I bought a CTEK one and keep all batteries always on charge over winter when not using the car.

CTEK are expensive but there are penty of alternatives and some highly rated.
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Re: Car Battery failures

Post by ChrisJC »

I do do that - I have a nice charger, and I try to use it. But sometimes I think I will be going out again in a few days and don't bother, then 2 weeks later the car is dead!

I am just trying to understand why the batteries are failing.

Chris.
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Re: Car Battery failures

Post by DaveEFI »

The alternator won't stuff its full output into a battery. The maximum voltage ii can produce is limited to about 14.5 volts. That voltage is the same for a 50 amp or 150 amp alternator. And as the battery state improves, the charge rate in amps comes down. I think you'll find the maximum charge the alternator produces is about 30 amps. Large alternators are there to cope with heavy loads, not to charge a battery faster.

Letting a lead acid battery go flat even once shortens its life dramatically. A very expensive thing to do.

Two options. On a little used car, disconnect the battery. A good battery self discharges very slowly. Reconnect it after 6 months, and it will still start the car. Of course plenty will say "but it will lose the radio memory, etc" Yes it will - just as happens if it is allowed to go flat.

Other is to use a mains charger with a maintenance feature. Plenty on the market for CTEK etc, but Lidl and Aldi have then from time to time at about £14. If this is a regular thing, worth installing one in the car, using a waterproof mains inlet. Making it easy to keep the car secure when on charge, like say on a driveway.
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Re: Car Battery failures

Post by DaveEFI »

ChrisJC wrote:
Mon May 03, 2021 9:01 am
I do do that - I have a nice charger, and I try to use it. But sometimes I think I will be going out again in a few days and don't bother, then 2 weeks later the car is dead!

I am just trying to understand why the batteries are failing.

Chris.
Most cars are happy with 3 weeks or more unused. Two weeks suggest you have a fault. You need to measure the current draw from the battery with the car sleeping. I'd expect it to be about 30 mA. (O.03A)
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Re: Car Battery failures

Post by stevieturbo »

A decent charger will also have a battery recon mode, where it de-sulphates the battery or something via a specific charging method

I've a little C-Tek that does this, and I've used it a couple of times to good effect, so always worth a try first.

But if it will be sitting up, disconnect the battery in the car, and either leave a suitable charger on it, or just leave it disconnected. Running batteries totally flat will kill them.
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Re: Car Battery failures

Post by ChrisJC »

I will probably disconnect the battery in the Landie as that has no side effects. However disconnecting the battery in a P38 Range Rover is a complete pain as you have to do it quickly to avoid setting off the alarm, and when you reconnect you have to recalibrate all of the windows and sunroof!

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Re: Car Battery failures

Post by ChrisJC »

DaveEFI wrote:
Mon May 03, 2021 9:13 am
The alternator won't stuff its full output into a battery. The maximum voltage ii can produce is limited to about 14.5 volts. That voltage is the same for a 50 amp or 150 amp alternator. And as the battery state improves, the charge rate in amps comes down. I think you'll find the maximum charge the alternator produces is about 30 amps. Large alternators are there to cope with heavy loads, not to charge a battery faster.
Not sure I agree with that Dave. The alternator can in theory deliver up to 150Amps at 14.4Volts. If the battery is flat, I don't know if the battery will take that sort of current, or if the voltage will shoot up to 14.4 and the current will fall. I can't find any data to show how much current a completely dead battery will draw if 14.4V is put across the terminals.

The alternator doesn't know if it's charging a battery or driving the a/c, air suspension and ABS pumps, both electric fans, wipers front and rear, heated seats, heated windscreen and heated rear window. All it does it tries its hardest to deliver 14.4V by increasing its current output until it gets to 14.4V or 150A. If one of those loads is a flat battery, it doesn't know or change it's behaviour. It's just a perfect voltage source.

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Re: Car Battery failures

Post by DaveEFI »

Thing is a 28 amp dynamo will produce 14.5v when charging. That is the sort of figure given for all charging systems regardless of output.
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Re: Car Battery failures

Post by garrycol »

ChrisJC wrote:
Mon May 03, 2021 12:35 pm
Not sure I agree with that Dave. The alternator can in theory deliver up to 150Amps at 14.4Volts. If the battery is flat, I don't know if the battery will take that sort of current, or if the voltage will shoot up to 14.4 and the current will fall. I can't find any data to show how much current a completely dead battery will draw if 14.4V is put across the terminals.
The alternator does not force its current into a battery - it is the battery that determines what current it needs up to what is available. So when the battery is flat but still alive (not dead) it will take only what it needs, obviously higher at the start and as its voltage rises it needs less amps so will take less from the charger/alternator.

While the charge from an alternator is not ideal, it is not the charge that kills the battery, it is the fact it was flat in the first place.

Now smart chargers are more precise but they normally what in as much voltage and amps into the battery as quick as possible - when the battery gets to about 80% charge it switches to an absorption charge where voltage remains steady and amps start to drop off - at some point near fully charged the charger see the battery is charged and provides 13.2v with amps dropping off.

I would charge you battery on a smart charger to get best results overall but an alternator will still do the job.

Garry

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Re: Car Battery failures

Post by ChrisJC »

garrycol wrote:
Mon May 03, 2021 2:00 pm
The alternator does not force its current into a battery - it is the battery that determines what current it needs up to what is available. So when the battery is flat but still alive (not dead) it will take only what it needs, obviously higher at the start and as its voltage rises it needs less amps so will take less from the charger/alternator.
I don't agree. The alternator will increase it's current output until either it reaches 14.4V, or 150A. The battery will determine which of those happens first.

With a nearly full battery, it will reach 14.4V way before 150A is reached, and when the battery is full, it will be at 14.4V with practically zero current.

However, I suspect that when the battery is flat, it takes well over 15A without reaching 14.4V. I see I am going to have to do some measurements on a flat battery.

Let us imagine you had a super-duper battery charger with 1000A on the scale. If you connected it to a flat battery, what do you think the current would read?

Chris.
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Re: Car Battery failures

Post by stevieturbo »

ChrisJC wrote:
Tue May 04, 2021 7:24 am


Let us imagine you had a super-duper battery charger with 1000A on the scale. If you connected it to a flat battery, what do you think the current would read?

Chris.

Pretty sure it would not read 1000A

But likewise I do not believe that an alternator will put maximum output into a flat battery....but I've also never tested it.
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Re: Car Battery failures

Post by DaveEFI »

stevieturbo wrote:
Tue May 04, 2021 8:46 pm
ChrisJC wrote:
Tue May 04, 2021 7:24 am


Let us imagine you had a super-duper battery charger with 1000A on the scale. If you connected it to a flat battery, what do you think the current would read?

Chris.

Pretty sure it would not read 1000A

But likewise I do not believe that an alternator will put maximum output into a flat battery....but I've also never tested it.
Quite. There is a lot more going on with an alternator regulator.

Most car charging systems settle at 13.8v when the battery is fully charged. A fully charged battery is quoted as 13.2v. So a differential of 0.6v. The internal resistance of a good battery is quoted as 0.001 ohms. So Ohm's law says a current of 600 amps is flowing.

With a large car battery of say 70 amp/hr, and a 100 amp alternator, figures would suggest an hour's running would fully charge a flat battery. It doesn't.
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Re: Car Battery failures

Post by jp928 »

If a battery flattens in 2 weeks idle there is a significant drain somewhere. Disconnect one pole and put a meter across the pole to the lead set to amps. if you see more than ~30ma, pull fuses one at a time until the reading drops, then you know where some of the drain is - but there may be more than one, so keep looking.
Separately its a good idea annually, especially in a moist climate, to locate ALL the earth/ground connections, undo/disconnect them, clean all the surfaces, apply something like Deoxit, reconnect. Also do the battery poles - clean to bright, lightly grease (slows corrosion growth). Clean the alternator connections.
My P6B with a 2 year battery has no trouble starting after a month idle, but its only driving a clock. I suspect a P38 will have more standing drain than any earlier Landy. Get a trickle charger, and find a simple convenient way to connect it - I have a Merit connector socket under a rear number plate on one car so I dont have to open anything to connect/disconnect it. Hide a socket in the grille?
https://www.jaycar.com.au/merit-panel-s ... MeEALw_wcB

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